I went through the same process last year on coilovers, and made the decision to change everything affecting it.
I bought the AP Sportfahrwerke Coilovers , had a look at them at AMD, good German quality.
As my car is mainly for track days getting the set up and balance was important. Done this many times but not on a Z3. Please bare in mind though this setup is excellent on the road, compliant and very pointy/grippy as the springs and dampers are doing all the work, not the bushes and body flexing.
spec as follows:-
AP coilovers, but changed the front springs to 350lbs with no assistors and different coil caps
Adjustable rose jointed top mounts, with negative camber on front
Polybused all round
E-tech strut brace
new wishbones (ie all ball joints are new)
new drop links
modified rear ARB, to help with location
Corner weights balanced as well 52/48
the neg camber on the front helps turn in no-end
the car is well balanced and handles very neutral
i am also running 18" wheels, with 225's all round for balance. no tramlining, perfect
hope this helps